Recently, as a portable electronic device has widely spread, in accordance with rapid miniaturization, lightness, and thinness of the portable electronic device as described above, in a battery, which is a power supply of the portable electronic device, development of a secondary battery capable of having a small size and having a light weight, and being charged and discharged for a long period of time while having excellent high rate capability has been urgently demanded.
Among the currently applied secondary batteries, a lithium secondary battery developed in the early 1990s has been spotlighted due to advantages such as a high operation voltage and significantly high energy density as compared to conventional batteries using an aqueous electrolyte such as a Ni-MH battery, a Ni—Cd battery, and a lead sulfate battery, and the like. However, in the lithium secondary battery as described above, there are safety problems such as ignition and explosion, and the like, caused by using a non-aqueous electrolyte. As a capacity density of the battery is increased, this problem becomes more severe.
In a non-aqueous electrolyte secondary battery, there is a serious problem such as safety deterioration of the battery generated at the time of continuous charge. One of the causes affecting safety of the battery is heat generation due to collapse of a cathode structure. An operation principle thereof is as follows. That is, a cathode active material of a non-aqueous electrolyte battery is composed of lithium, a lithium containing metal oxide capable of intercalating and releasing lithium ions, and/or the like, and as a large amount of lithium is detached at the time of over-charge, a structure of the cathode active material as described above is changed to a thermally unstable structure. In this over-charge state, when a battery temperature reaches a critical temperature due to external physical impact, for example, exposure to a high temperature, or the like, oxygen is released from the cathode active material having an unstable structure, and the released oxygen generates an exothermal decomposition reaction with an electrolyte solvent, or the like. Particularly, since combustion of the electrolyte is further accelerated by oxygen released from a cathode, the battery may be ignited and ruptured due to thermal runaway caused by a series of exothermic reactions as described above.
In order to suppress the above-mentioned ignition or rupture due to an increase in a temperature in the battery, a method of adding an aromatic compound to the electrolyte as a redox shuttle additive has been used. For example, a non-aqueous lithium ion battery capable of preventing over-charge current and a thermal runaway phenomenon caused by the over-charge current by using an aromatic compound such as biphenyl has been disclosed in Japanese Patent No. 2002-260725. In addition, a method of improving safety of a battery by adding a small amount of an aromatic compound such as biphenyl, 3-chlorothiophene, or the like, to increase an internal resistance by electrochemical neutralization in an abnormal over-voltage state has been disclosed in U.S. Pat. No. 5,879,834.
However, in the case of using the additive such as biphenyl, or the like, there are problems in that when a relatively high voltage is locally generated in a general operation voltage, the additive is gradually decomposed during a charge and discharge process, or when the battery is discharged at a high temperature for a long period of time, an amount of biphenyl, or the like, may be gradually decreased, such that safety may not be secured after 300 charge and discharge cycles. In addition, there is a problem in storage characteristics, or the like.
Meanwhile, as a method of increasing an electricity charge amount for a small size and high capacity of the battery, a high voltage battery (4.4V system) has been continuously studied and developed. Even in the same battery system, when a charge voltage is increased, a charge amount is generally increased. However, safety problems such as decomposition of the electrolyte, a shortage of a space for lithium intercalation, a risk due to a potential rise of an electrode, or the like, may occur. Therefore, in order to manufacture a battery that may be used at a high voltage, overall conditions are managed with a system so that a large standard reduction potential difference between an anode active material and a cathode active material may be easily maintained, and an electrolyte is not decomposed at this voltage.
Considering this point of the high voltage battery, it may be appreciated that in the case of using existing over-charge preventing agents such as biphenyl (BP) or cyclohexylbenzene (CHB) used in a general lithium ion battery, even during a normal charge and discharge operation, large amounts of these over-charge preventing agents may be decomposed, and characteristics of the battery may be rapidly deteriorated even at a slightly high temperature, such that a life cycle of the battery may be decreased. Further, in the case of using a generally used non-aqueous carbonate based solvent as an electrolyte, when a battery is charged at a voltage higher than 4.2V, which is a general charge potential, oxidizing power may be increased, such that as charge and discharge cycles are performed, a decomposition reaction of the electrolyte is carried out, such that life cycle characteristics may be rapidly deteriorated.
Therefore, a method for improving safety and capacity of a battery at the time of high temperature storage without deteriorating life cycle characteristics of a high voltage battery (4.4V system) has been continuously demanded.